1. Gypsum and anhydrite

Calcium sulfate is found in nature in two main forms:

• hydrated with two molecules of water per molecule of calcium sulfate (CaSO ₄ . 2H ₂ O): this is gypsum ;

• anhydrous (CaSO ₄ ): this is anhydrite.

It can also be found, in certain specific geological situations, hydrated with a fraction of one water molecule per molecule of calcium sulfate: this is bassanite (CaSO ₄ . 1/2 H ₂ O), metastable under normal conditions and only found in small quantities (less than 1% by mass) in gypsum.

Gypsum and anhydrite also exist as by-products...